These applications have been commonly assigned to Xtera Communications, Inc.
The present invention relates to the field of communication systems, and more particularly to a method and system operable to reduce degradation of an optical signal to noise ratio when multiple communication bands are communicated over a common optical link.
Conventional optical networks have traditionally communicated information over the conventional communication band (C-Band), which is typically identified as including wavelengths between approximately 1520 and 1560 nanometers. As optical communication service providers continually strive to increase system bandwidth, some systems have begun to investigate expanding system bandwidth by communicating information over the long band (L-Band) residing at approximately 1565 to 1610 nanometers.
When optical signals carrying multiple wavelengths of light and even multiple bands of wavelengths are communicated over one or more spans of fiber, various phenomena can cause wavelength dependent attenuation, where some wavelengths are attenuated more than other wavelengths during transmission. For example, particular optical fibers may result in longer wavelengths being attenuated more than shorter wavelengths. In other cases, the stimulated Raman scattering effect can cause higher wavelength signal components tend to rob energy from lower wavelength signal components, effectively attenuating the lower wavelength signals relative to the higher wavelength signals.
The stimulated Raman scattering effect becomes more prevalent with increases in system power, system bandwidth, and the transmission distance or number of transmission spans in the system. Multiple band communication systems having several communication spans can, therefore, be particularly vulnerable to the detrimental effects of stimulated Raman scattering. In addition, the Raman scattering effect has in the past provided a strong disincentive to even attempting to use the short communications band (S-Band) residing at approximately 1490 to 1520 nanometers. Because these phenomena result in signals being attenuated while noise levels remain constant, wavelength dependent attenuation characteristics typically lead to degradation of the optical signal to noise ratio.
One approach to addressing problems associated with Raman scattering, at least in the C-Band, involves introducing into signal wavelengths of a single communication band a negative gain tilt that is exactly opposite the gain tilt introduced by Raman scattering. This approach seeks to flatten the output power spectrum at the system""s optical receivers. This approach carries a significant disadvantage, however, in that it does nothing to preserve the optical signal to noise ratio across the communication band, and often leads to degradation of that ratio, particularly in shorter signal wavelengths.
The present invention recognizes a need for a method and system operable to preserve an optical signal to noise ratio in optical transmission systems communicating signals carrying multiple wavelengths or multiple bands of signal wavelengths. In accordance with the present invention, apparatus and methods operable to reduce wavelength dependent attenuation and preserve optical signal to noise ratios are provided that substantially reduce or eliminate at least some of the shortcomings associated with prior approaches.
In one aspect of the invention, an optical communication system operable to reduce degradation of an optical signal to noise ratio where signals having multiple wavelengths are communicated over a common optical link comprises an amplifier assembly operable to introduce to a lower communication band a first gain and to introduce to a higher communication band a second gain that is different than the first gain. The amplifier assembly is further operable to introduce a variable gain tilt into one or more of the communications bands. The different gains applied between bands and the gain tilt introduced into at least one of the bands results in a reduction of a loss of optical signal to noise ratio that could otherwise be caused by wavelength dependent attenuation when the communication bands are combined and communicated over an optical link.
In one particular embodiment, the first gain introduced into the lower band can be made larger than the second gain introduced in the higher band. In addition, a negative gain tilt can be applied to at least the lower band. This embodiment reduces loss of optical signal to noise ratio that could otherwise be caused by stimulated Raman scattering.
In another aspect of the invention, a method of reducing degradation of an optical signal to noise ratio where multiple communication bands are communicated over a common optical link comprises introducing a first gain into a lower communication band comprising a first plurality of signal wavelengths and introducing a second gain that is different than the first gain into a higher communication band. The higher communication band comprises a second plurality of signal wavelengths having longer wavelengths than the first plurality of signal wavelengths. The method further comprises introducing a gain tilt into at least one of the lower and higher communication bands. The differential gains applied between bands and the gain tilt introduced into at least one of the bands results in a reduction of a loss of optical signal to noise ratio that would otherwise be caused by wavelength dependent attenuation when the communication bands are combined and communicated over an optical link.
Depending on the specific features implemented, particular aspects and embodiments of the present invention may exhibit some, none, or all of the following technical advantages. One aspect of the invention facilitates communication of signals having wavelengths from multiple communication bands, while reducing degradation of an optical signal to noise ratio associated with those signals. In a particular embodiment, the invention can facilitate optimization of the optical signal to noise ratio across multiple wavelengths within a communication band and even across multiple communication bands.
One aspect of the invention advantageously reduces differences between optical signal to noise ratios associated with the lower and higher bands to improve the system""s transmission capabilities. In a particular embodiment, the invention can provide a relatively flat optical signal to noise ratio across wavelengths of each communication band, and even across multiple bands. Particular embodiments of the invention advantageously facilitate simultaneous use of multiple communications bands, even including the short communications band (S-Band) that had previously been severely impacted by Raman scattering effects.
In some embodiments of the invention, gain tilt can be introduced in whole in or part by preconditioning circuitry separate from the amplifier assembly or assemblies introducing the first and second gains. This aspect of the invention advantageously reduces or eliminates demands on the amplifiers to introduce gain tilt, or at least reduces the necessary range of the amplifiers.
One aspect of the invention facilitates tuning system elements, such as amplifiers, attenuators, and/or filters, to account for various network characteristics. In particular embodiments, the invention facilitates monitoring various network characteristics and dynamically tuning system elements to optimize, or at least reduce degradation of an optical signal to noise ratio.
Other technical advantages are readily apparent to one of skill in the art from the attached figures, description, and claims.